One thing mentioned on the responses that might be really concerning is the fact that this will mean that for a lot of analytics it will look like firefox usage is close to zero. No usage data, devs caring less about firefox, users having more problems when using firefox, less users using firefox, less users having 3rd party trackers blocked, chrome monopoly growing.
It's enough that a few large websites provide stats summary for their users. It is not necessary that Google, FB and co. track the entire Internet.